The invention relates to a dye laser for generating laser pulses of long duration. The invention particularly relates to a gas discharge lamp-excited high-power dye laser with a long pulse intended e.g. for medical use.
Dye lasers are frequently used in several medical applications, for example in treatment of different vascular lesions by means of selective photothermolysis. Vascular lesions that are treatable with a dye laser include port wine stains, hemangiomas, small varicose veins, facial spider veins, rosacea, disturbing surface veins and scar tissue. In these applications, and especially if the scope of application is to be extended to cover the treatment of vascular lesions formed by blood vessels thicker than 1 mm, laser pulses with a relatively long duration and relatively high energy are necessary. The typical pulse duration achievable with a dye laser is approx. 1 to 1.5 ms and the energy density a few, or at most a few dozens of joules per square centimeter. In the treatment of some vascular lesions a longer pulse duration would, however, be necessary if good results are to be achieved. Dye laser arrangements in which a longer pulse duration has been achieved are known for example from U.S. Pat. Nos. 5,624,435 and 5,746,735.
In the arrangement according to U.S. Pat. No. 5,624,435 a longer pulse has been achieved by changing the lasing dye solution during lasing, in which case the chemical reactions occurring in the dye during lasing, such as photodegradation, which at some point stop lasing, are not as restrictive as in conventional devices in which the dye is not changed during lasing. In the arrangement of the patent in question it is, however, necessary to use high-capacity, high-speed dye pumps which render the device difficult to control and expensive.
U.S. Pat. No. 5,746,735 discloses an arrangement in which the beams of two or more separate laser heads are combined by means of optics so that they can be focused on the target to be treated along one transmission path, such as an optical fiber. The long pulse duration is achieved by triggering the laser heads successively in sequence. This method is very expensive and complicated, since each laser head is an independent unit which needs to be provided with complete resonator optics including the wavelength-selective elements. The problems are emphasized especially when pulses longer than 2 ms are sought for. Since one laser head can generate a pulse with a duration of approx. 1 ms, the generation of a pulse of 10 ms would require 10 laser heads, i.e. 10 separate optic resonators and very complicated and low-efficiency combining optics of beams. Thus the costs of such an arrangement will be very high and its efficiency poor.